The Solar System and the Planet Earth 305
Cloud is believed to be a spherical region of comets that extends out to
50,000 AU. Although it has never been directly observed it is believed to
be the source of the long period comets.
A comet is composed of meteoric material embedded in the ices of
methane, ammonia and water. It has been postulated that gravitational
perturbation by the planets causes some of these objects to change their
stable orbits beyond Neptune into the highly elliptic orbits, which bring
the comets close to the Sun.
As a comet passes close to the Sun, the coma and the tail begin
to form from the material evaporated by the Sun’s radiation. The coma
is a spherical shell of gas surrounding the nucleus of the comet. Its
size, which is of the order of 100,000 kilometers depends on how close
to the Sun the comet approaches — the closer the approach, the larger
the coma. The tail, which is frequently curved consists of both gas
and dust and grows as the comet approaches the Sun reaching lengths of
up to 10 million km. The tail does not follow the comet but rather always
faces away from the Sun so that at certain times the comet follows its tail
The reason for this is believed to be due to the forces on the gas particles
of the tail generated by the solar wind and the radiation pressure of the
Sun’s light.
During each passage of the comet close to the Sun, it loses
approximately 1/2 of 1% of its mass, which means after 200 passages or
so the comet disintegrates. The rate of mass loss actually depends on
how close the comets pass to the Sun so that it is possible for a comet to
break up after only a few passes past the Sun. The distance of the closest
approach is usually somewhere between the orbit of Venus and Mars
although one comet is known to have come so close to the Sun that it
passed through its corona.
Centaurs
Centaurs are unstable minor planets that lie somewhere between Jupiter
and Neptune and have both asteroid and comet like behaviours, hence
their name. The first to be discovered was 944 Hidalgo and the largest
centaur is 10199 Chariklo with a diameter of 260 km.
Meteors, Dust and Gas
Meteors are chunks of matter made of stone or metals, which revolve
around the Sun. The stony meteors are made of rocks like those found on